1. Field of the Invention
The present invention relates to a solid golf ball and, more particularly, to a solid golf ball having improved resilience on shot, flight performance and shot feeling.
2. Description of the Prior Art
Golf balls are classified into two categories. The one is a thread-wound golf ball which is obtained by winding a rubber thread around a center filled with a solid or liquid and then covering the rubber-thread wound center with a cover mainly made of balata or synthetic resin. The other one is a solid golf ball such as a one-piece golf ball and a multi-piece golf ball. A multi-piece golf ball is formed by covering a solid core with a thermoplastic resin cover. The solid core is a homogeneous solid spherical body, or consists of a central part and a layer or a plurality of layers placed on the central part (i.e. layered structure). The cover has a one- or multi-layered structure. A solid golf ball has been becoming popular because of its excellent flying distance and excellent durability, however a solid golf ball is inferior in shot feeling to a thread-wound golf ball because it is harder than a thread-wound golf ball.
The core of a conventional solid golf ball is produced by vulcanizing a rubber composition containing a polybutadiene rubber as a base rubber, a metal salt of an unsaturated carboxylic acid as a co-crosslinking agent, zinc oxide as a vulcanizing activator and a weight adjuster, an organic peroxide (e.g. dicumyl peroxide) as a vulcanizing initiator and other additives. The organic peroxide generates radicals and the radicals causes a crosslinking reaction of polybutadiene with the metal salt of an unsaturated carboxylic acid, the resulting solid golf ball has a proper hardness and durability and enhanced resilience. Although high resilience may give a long flying distance, hitting the solid golf ball having the hard core causes a large impact. A large impact on hitting gives an impression that it is difficult for unskilled or women golfers to hit the solid golf ball.
Proposals put forward for improving shot feeling by reducing the hardness of a solid golf ball. However, when the hardness of the core or cover is reduced, other problems occur, that is, the resilience of the solid golf ball is decreased and flying distance is shorter.
Japanese Unexamined Patent Publication Nos. 2-297384, 4-109970 and 9-122273 disclose solid golf balls whose cores are formed by vulcanizing a rubber composition containing an organosulfur compound. According to these disclosures, the solid golf ball may keep high resilience without increasing the hardness of the ball by adding the organosulfur compound. These disclosures indicated merely that the amount of the organosulfur compound based on the weight of the base rubber, however shot feeling and resilience is affected by not only the amount of the organosulfur compound based on the weight of the base rubber but also degree of crosslinking and the conditions of crosslinking in the core.
Under these circumstances, the present invention has been accomplished. An object of the present invention is to provide a solid golf ball giving excellent shot feeling without impairing intrinsic resilience of the solid golf ball.
In order to fulfil the above object, the present inventors have paid attention to the fact that shot feeling and resilience of the golf ball containing a core made from the vulcanized rubber has a close relation with the degree of crosslinking in the core and have found that the amount of an sulfide must be determined by taking the amount of the crosslinking system (base rubber, organic peroxide and xcex1, xcex2-unsaturated carboxylic acid or metal salt thereof) into consideration, thus completing the present invention.
According to an aspect of the present invention, a solid golf ball comprises a core being entirely made form a vulcanized rubber composition, and a cover covering the core. The rubber composition which contains 100 parts of a base rubber, A parts by weight of an xcex1, xcex2-unsaturated carboxylic acid or a metal salt thereof, B parts by weight of an organic peroxide, and C parts by weight of a sulfide, said A, B and C satisfying the following relationship:
0.020xc3x97Axe2x89xa6B+Cxe2x89xa60.050xc3x97A
and A being in the range from 25 to 45.
Another solid golf ball of the present invention comprises a core consisting of a central part and a layer or a plurality of layers placed on said central part, and a cover covering said the core said central part or at least one layer being made from the vulcanized rubber composition as described above.